Polymer blends have widespread technological importance because they offer a strong analogy to the copolymerization as blends are also a means of combining the unique properties of the constitutive polymer species. However, Paul Flory, Nobel Prize winner in polymer chemistry, has commented that “ . . . incompatibility of chemically dissimilar polymers is observed to be the rule and compatibility the exception.” Therefore, there has been a tremendous amount of research and development in block and graft copolymers because these copolymers are generally regarded as chemically linked homopolymers. These materials show good physical properties but they tend to be more expensive, often requiring an additional hydrogenation step to improve the polymer stability. Therefore, there is always a need to make chemically linked polymers that exhibit some of the properties of the component polymers in an economic way.
These chemically linked polymers may have improved melt elasticity and processability, and may be used as a compatibilizer for a polymer blend, thereby improving its properties. Varied applications exist for such property improvements, for example, blown film and thermoforming applications. Billions of pounds of polyethylene (PE) are annually processed by blown film extrusion techniques to produce grocery sacks and trash can liners. Linear low density polyethylene (LLDPE), in particular, shows good bubble stability and the resulting film is suitable for many applications. Overall, LLDPE has a narrow molecular weight distribution (MWD=Mw/Mn) and better down-gauging and optical properties, however impact strength is not optimal. Also, there are many challenges in the film blowing process of LLDPE. Most LLDPEs have higher MWs but narrower MWDs, and the short chain branching (SCB) of LLDPEs does not usually provide shear thinning. LLDPE is therefore undesirably viscous at the high shear rates when processed in an extruder and die. The more viscous LLDPE may also increase screw torque, barrel wear, melt temperature, and occurrences of sharkskin at the die exit. At the same time, LLDPE typically shows lower extensional stresses at low strain rates occurring in the molten tube and bubble inflation regions, resulting in a higher chance of bubble instability. One of the primary objectives of the blown film process is to produce a thin film with a uniform gauge and good optical and mechanical properties, and to avoid any gels and foreign and unmelted materials in the extruded, drawn film. Accordingly, the blown film industry continues to have a need for a polyethylene material with a balanced melt strength and drawn speed to break, with the capability of being shear thinned, and free of gels.
In the plastic fabrication industry, a variety of resins may be used, and typically include: polypropylene (PP), general purpose polystyrene (GPPS), polycarbonate (PC), acrylonitrile-butadiene-styrene (ABS), and nylon (also known as polyamide, or PA). Injection molding is one of the primary plastic fabrication techniques for rapidly creating large quantities of plastic articles ranging from disposable food containers to high precision engineering components. This plastic fabrication method is distinguished from others by using injection and a hollow mold form to shape the final article. Polymers exhibiting shear thinning behaviors are highly desirable because they can be easily processed in high shear rate fabrication methods, such as injection molding. Accordingly, there is a need for polymers and polymer blends that exhibit good shear thinning behavior in injection molding fabrication processes.
Thermoforming is the process of heating a solid plastic article, mostly in the sheet form, to a temperature where it softens but does not flow, then reshaping it. This process has a large cost advantage over injection molding because of the less expensive mold and lower energy consumption. Thin-gauge sheet or film is used in thermoforming to produce disposable/recyclable food, medical and general retail products such as containers, cups, lids, and trays. Thick-gauge sheet is used to produce larger, usually more permanent, items such as plastic pallet, truck beds, and spas. The buildup of slack in the heated sheet, however, needs to be avoided in the thermoforming process. Also, it is desirable to thermoform polyolefins having high melt strength or melt elasticity. Accordingly, there is a need for materials with improved melt strength or elasticity, and improved sagging resistance for applications such as thermoforming.
U.S. Pat. No. 7,338,994 discloses rheology-modified, gel-free thermoplastic elastomer compositions comprising a melt blend of an ethylene/α-olefin polymer and a high melting polymer such as polypropylene or a propylene/α-olefin copolymer wherein the rheology modification is induced by a combination of a peroxide and a free radical coagent in a peroxide:coagent ratio of 1:4 to 1:20 and a maximum peroxide concentration of 0.075 wt %. The resulting compositions have an elastomeric phase, a non-elastomeric phase, and certain physical properties that exceed those of a like composition that is rheology-modified by peroxide alone or peroxide and coagent in a 1:2 to 2:1 ratio. The compositions are used to make a variety of articles of manufacture, such as tubing and weather-stripping, via calendaring, extrusion, and molding techniques such as blow molding.
There remains a need for new chemically linked polymers, and blends thereof, that exhibit some of the properties of the component polymers, having utility in various applications such as blown film and thermoforming.